Ghilliebear (PCS) made a nice rig for me a couple of years ago. It's a 51K with a cut-down .22lr conversion. In fact, it is a dedicated .22lr and even marked as such - has some other neat markings as well. (I know, no pix so it must not exist).
I have about 8 HK magazines, which are the only ones that work well in full-auto. I also have one of the unicorn drum adaptors but I have yet to use it.
I had waited for years for someone to put out a host and finally went this route though I had not ruled out chopping up one of my other .22 conversions and having a cut-down 33/93 platform in hopes of using existing BDM .22lr AR magazines. (The G3 .22lr conversion is so much cheaper)
I have definitely spent ton's of time perfecting it's functioning and the most current lingering issue is in the feeding of rounds from a fully loaded magazine (20). It will run all day with 17 or 18 loaded but 20 seems to be a recurring issue so my tinkering is now focused on the magazine springs.